Two Port Network

INTRODUCTION

A pair of terminals through which a current may enter or leave a network is known as a port. Most of the circuits we have dealt with so far are two-terminal or one-port circuits, represented in fig. (a). We have considered the voltage across or current through a single pair of terminals-such as the two terminals of a resistor, a capacitor, or an inductor. We have also studied four-terminal or two-port circuit involving op-amps, transistors and transformer as shown in fig. (b). Thus, we can say that a two-port network is an electrical network with separate ports for input and output.
In general, a network may have n ports. In this chapter, we are mainly concerned with two-port networks (or simply, two ports).

Network Parameters

In two port network four variables are use (i.e., V1, V2, I1, I2) out of four variable, we can chose two variable in six different way (4C2), and hence we have six sets of two-port.

• Z-parameters (Impedance parameters)
• Y-parameters (Admittance parameters)
• ABCD parameters (Transmission parameters)
• A′ B′ C′ D′ parameters (Inverse transmission parameters)
• h-parameters (Hybrid parameters)
• g-parameters (Inverse hybrid parameters)

Impedance or Z-parameters (Open circuit parameters)

V₁ = Z₁₁I₁ + Z₁₂I₂ and V₂ = Z₂₁I₁ + Z₂₂I₂

Admittance or Y-parameters (Short circuit parameters)

I₁ = Y₁₁V₁ + Y₁₂V₂ and I₂ = Y₂₁V₁ + Y₂₂V₁

h-parameters (Hybrid parameters)

V₁ = h₁₁I₁ + h₁₂V₂ and I₂ = h₂₁I₁ + h₂₂V₂

g-parameters (Inverse hybrid parameters)

I₁ = g₁₁V₁ + g₁₂I₂ and V₂ = g₂₁V₁ + g₂₂I₂

ABCD parameters (Transmission parameters)

V₁ = AV₂ – BI₂ and I₁ = CV₂ – DI₂

Conditions for a network to be symmetrical and reciprocal